Exploding vesicles

Vesicles are bilayer membrane structures that encapsulate an internal aqueous compartment. Fatty acid vesicles have been studied as models of primitive cell membranes at the origin of life [1-7], and phospholipid vesicles (liposomes) have been widely studied as models of modern cell and organelle membranes [8,9] and as drug delivery vehicles [10-12]. We first observed the phenomenon of "exploding vesicles" during microscopic observations of large (approximately 4 μm in diameter) oleate vesicles containing 10 mM HPTS (8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt, a water-soluble, membrane-impermeable fluorescent dye), in 0.2 M Na-bicine buffer, pH 8.5. The vesicles were prepared by extrusion and dialysis, so that the fluorescent dye was present only inside the vesicles while the buffer was present in both inner and outer solutions [13]. To our surprise, we observed that these vesicles suddenly exploded shortly (approximately 0.5 sec) after being exposed to intense illumination from a metal halide lamp (estimated irradiance 2.5 W/mm2), and released their encapsulated dye along with smaller internal vesicles (Figure 1A; Additional File 1 Figure S1; Additional File 2). The actual vesicle rupture appeared to take place in < 2 ms (3 frames in a recording from a high-speed camera: Additional File 3); this estimate is an upper limit because of the time required for diffusion of the released dye away from the vesicle. We observed similar vesicle explosions using vesicles containing different internal fluorescent dyes, such as calcein and Rose Bengal (a photodynamic therapy drug).To distinguish between physical rupture and rapid permeabilization of the vesicle membrane, we labeled the vesicles with a membrane-localized dye (Rh-DHPE, Lissamine rhodamine B 1,2-dihexadecanoyl-sn-glycero-3-phosphoethanolamine). Upon illumination, we observed that the vesicle membrane burst open on one side and then quickly recoiled (Figure 1B; Additional File 4). When using slightly lowe